package jdbc;

import java.sql.Connection;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;

/**
 * 聚合函数
 * SELECT COUNT(*) count
 * FROM class c
 * JOIN student s ON c.id = s.class_id
 * WHERE c.name='1年级1班'
 */


public class JDBCDemo7 {
    public static void main(String[] args) {
        //查看1年级1班一共多少人
        /*
            SELECT COUNT(*) count
            FROM class c
            JOIN student s ON c.id=s.class_id
            WHERE c.name='1年级1班'
         */
        try (Connection connection = DBUtil.getConnection();){
            Statement statement = connection.createStatement();
            String sql = "SELECT COUNT(*) count " +
                    "FROM class c " +
                    "JOIN student s ON c.id=s.class_id " +
                    "WHERE c.name='1年级1班'";
            ResultSet rs = statement.executeQuery(sql);
            if(rs.next()){
                int count = rs.getInt("count");
                System.out.println("一共:"+count+"人");
            }

        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}
